CDL Vehicle Combination Practice Test

What is the first step when uncoupling a trailer?

1. Lock the trailer brakes

2. Shut off the trailer air supply

3. Detach the electrical line

4. Remove the kingpin

The correct answer is: Shut off the trailer air supply

When uncoupling a trailer, shutting off the trailer air supply is a critical first step. This action helps ensure that the trailer’s brakes are not inadvertently activated when the tractor and trailer are separated. The air supply controls the application of brakes on the trailer, and by shutting it off, you prevent potential issues that could arise from brake engagement during the uncoupling process. Locking the trailer brakes is also important in the uncoupling process but typically comes after shutting off the air supply. This is to ensure that the trailer is stable and won't roll away during the uncoupling. Detaching the electrical line is another step that follows the air supply shutoff; it is essential to completely disengage the trailer's electrical connections. Similarly, removing the kingpin is one of the last actions taken to finalize the uncoupling process; it’s what separates the tractor from the trailer. By following the correct order of operations, safety and efficiency are maintained, making it crucial to first disable the air supply before proceeding with other tasks.